This invention relates generally to molds used for the production of golf balls using injection or compression molding processes, and it also relates to methods incorporating the use of these molds. The molds and related methods allow for greater ease in ball manufacture and allow for golf balls having superior properties.
Golf balls generally include a spherical core and one or more layers placed over the core. These cores can be made from a single material or can themselves include multiple layers of core material. These cores can be made using either compression or injection molding processes. Layers are placed over the core using a number of methods, including compression molding and injection molding.
Injection molding generally involves using a mold having one or more sets of two hemispherical mold sections that mate to form a spherical cavity during the molding process. The pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections can be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. The mold sections are connected to openings, or gates, evenly distributed near or around the parting line (i.e. point of intersection) of the mold sections through which the material to be molded flows into the cavity. The gates are connected to a runner and a sprue that serve to channel the molding material through the gates. The mold generally also includes vent pins located adjacent to the upper and lower poles of the spherical cavity formed by the mold sections. These vent pins are configured to retract from the mold sections to allow for air and other gases that build during the injection process to flow out of the cavity through the voids left by the retracted pins. The mold also includes ejection pins configures to extend from the mold sections into the cavity to push against the molded part and remove it from the mold. When used to mold a layer onto an existing structure, such as a ball core, the mold also includes a number of support pins (also known as core pins), disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical surface cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections.
One example of use of injection molding in preparing golf balls is in molding of a layer over a ball core. First, the core is placed inside a set of mold sections, and the mold sections are closed to form a spherical cavity around the core. The core is supported centrally in the cavity by the support pins, which are in an extended position to tightly engage the core. The mold sections and core are sized and placed to center the core in the shell cavity between the core and the inner surface of the mold sections. Then, a molten polymeric material is injected into the mold cavity under pressure through the gates. This polymeric material can be thermoplastic, or chemically reactive; that is, the material can begin to react during or after molding and form crosslinks to harden. When the space is mostly, but not completely, filled with the material, the support pins are retracted. At this point, the material in the cavity is sufficient in amount and viscosity to hold the core in place without the support pins. Injection of molten material continues, so that the material flowing through the gates fills the voids left by the retracted support pins. Eventually, the flow-fronts formed by the flows through each gate meet to form a complete layer over the core. The vent pins are retracted to provide venting paths to allow air and other gases in the cavity to flow out of the mold. The molded material layer is allowed to cool and hardened, and the layer and core are then removed from the mold by extending the ejection pins to push the core and molded layer out of the mold.
Another known method of forming a layer over a ball core involves use of both compression and injection molding. First, injection molding is used to form hemispherical half-shells. These are formed by using injection apparatus in which the mold sections are configured to form cavities that are hemispherical, rather than spherical. After the half-shells are formed, they are placed over a ball core in a compression molding apparatus. In the compression molding apparatus, the half-shells are subjected to heat and are pressed together along their equatorial edges to fuse and form a spherical layer over the core.
Molds currently used in molding processes for golf balls are made from aluminum or other conventional metal materials, such as 4140, P-20, or HR 13 steels. These materials are readily worked to produce the necessary structures. Though molds made from these materials are adequate for producing balls of commercial quality, they introduce certain imperfections and production difficulties. For example, during the injection process, weld or knit lines are formed when the flows of molten molding material do not fuse completely. These lines can occur due to various factors, such as non-uniform mold wall thicknesses, unusual flow paths of the molten material, pins blocking flow of material, or positioning of gates. Also, lines can form at the points of contact of the flow-fronts through the gates. Additionally, when material fills the space left by the retracting support pins, the material already has cooled to some degree, and the flow fronts of the material meet in this space to form knit lines. These lines are areas of weakness in the layer that are prone to fracture or crack. Knit line formation, and the subsequent weakness, is worsened when venting from the mold is insufficient to allow for the trapped gases to quickly exit. In addition to causing the marks discussed above, the compressed gases can produce burn marks caused by trapped gas during molding. Another problem is that of short shots, in which a part of the mold is not completely filled before the resin solidifies. This can occur when trapped gases are in a dead-end position, and gas venting is insufficient, producing increased pressure in the cavity that keeps out resin. In compression molding, trapped gas caused by improper venting can result in uneven distribution of the plastic melt between the core or mantled core insert and the cavity wall. This can lead to a sink on the molded surface of the ball or the molded layer having uneven thickness.
In view of the above, it is apparent that superior molds for manufacture of golf balls are needed that allow for superior molding quality, improved process control, and improved ball durability under impact. The present invention fulfills this need and other needs, and provides further related advantages.
The present invention is embodied in a mold configured for use in preparing a portion of a golf ball from a ball material. The mold has a contact region configured to contact the ball material during preparation of the portion, and the contact region comprises porous metal. The porous metal preferably has porosity sufficient to allow gases to escape through the porous metal. The porous metal preferably has a porosity between about 5% and about 50% by volume, more preferably between about 5% and about 40%, most preferably between about 10% and about 30%. The porous metal preferably incorporates interconnected pores in its structure having diameters between about 3 and about 10 microns. The porous metal preferably may comprise aluminum or a metal alloy, such as stainless steel.
Molds within the scope of the present invention preferably are configured to form a core, an intermediate layer, or a cover layer of a golf ball. In a preferred embodiment, the contact region of the mold is a plurality of mold sections that removably mate to define a generally spherical cavity, the mold sections incorporating porous metal. In preferred embodiments, the mold includes retractable support pins attached to the mold sections projecting into the cavity to support an inner portion over which the outer ball portion is prepared, with the support pins incorporating porous metal. The mold also may include extendable ejector pins attached to the mold sections configured to extend into the cavity to eject molded ball portions from the mold, with the ejector pins incorporating porous metal. The mold also may include retractable vent pins attached to the mold sections configured to retract from the cavity to allow gases to exit the mold cavity, with the vent pins incorporating porous metal. Additionally, the contact region of the mold may include a sprue, runner, or gates configured to route ball material to flow into the cavity, with the sprue, runner, or gates incorporating porous metal. Molds within the scope of the present invention may be configured for use in an injection or compression molding process.
A particularly preferred embodiment of the present invention is a mold in which the contact region of the mold defines an interior space, in which the ball material is situated, and an exterior space, and the mold further includes a vacuum device configured to reduce the pressure in the exterior space to a value less than the pressure in the interior space. Another preferred embodiment includes a pressurizing device configured to increase the pressure in the exterior space to a value greater than the pressure in the interior space. Yet another preferred embodiment includes both a vacuum and pressurizing device.
The present invention also resides in a method for forming a portion of a golf ball, including: preparing a mold incorporating porous metal and configured for use in preparing a portion of a golf ball, in which the mold defines an exterior space having an exterior pressure and an interior space having an interior pressure, and the mold further includes a vacuum device configured to reduce the exterior pressure to a value less than the interior pressure; using the vacuum device to reduce the exterior pressure below the interior pressure sufficient to increase flow of gas through the porous metal from the interior space to the exterior space; and, placing a ball material into the mold to form the portion so that gas flows from the interior space to the exterior space.
The present invention also resides in a method for forming a portion of a golf ball, including: preparing a mold incorporating porous metal and configured for use in preparing a portion of a golf ball from a ball material, in which the mold defines an exterior space having an exterior pressure and an interior space having an interior pressure, and the mold further includes a pressurizing device configured to increase the exterior pressure to a value greater than the interior pressure; placing a ball material into the mold to form the portion; and, using the pressurizing device to increase the exterior pressure above the interior pressure to a value sufficient to improve release of the portion from the mold.
Other features and advantages of the present invention should become apparent from the following detailed description of the preferred embodiments.